This invention relates to filters for analyzing gas such as air, and more particularly relates to filters for monitoring particulate matter in ambient air, e.g., in accordance with the Environmental Protection Agency""s national ambient air quality standards.
There are a variety of protocols and systems for sampling gases such as air. For example, there are National Institute for Occupational Safety and Health (NIOSH) air sampling protocols, as well as methods for surveying ambient air quality in accordance with requirements established by the Environmental Protection Agency (EPA). Illustratively, as set forth in 40 C.F.R xc2xa7xc2xa7 50.1-50.12 with appendices A-N, the EPA has established standards for measuring inhalable particulate matter (PM), referred to as the xe2x80x9cPM10xe2x80x9d standard (for measuring particulate matter with an aerodynamic diameter of less than or equal to a nominal 10 micrometers) and the xe2x80x9cPM2.5xe2x80x9d standard (for measuring particulate matter with an aerodynamic diameter of less than or equal to a nominal 2.5 micrometers). These EPA standards require drawing air through a filter, collecting the suspended particulate matter on the filter over a sampling period, and determining the particulate matter gravimetrically. The collected particles, usually on polytetrafluoroethylene (PTFE) filters, are weighed using microbalances under constant specified temperature and relative humidity conditions.
For example, the EPA PM2.5 standard requires drawing ambient air at a constant volumetric flow rate through an inertial particle size separator (impactor) where the suspended particulate matter is separated for collection on a PTFE filter over the specified sampling period. The mass concentration of the particles is computed and expressed in micrograms per cubic meter of air (xcexcg/m3).
However, in accordance with the present invention, it has been discovered that some material of interest was not measured when the above-referenced protocols were carried out, e.g., the conventional gravimetric systems lacked the capability and/or sensitivity to collect some of the particles in the gas.
The present invention provides for ameliorating at least some of the disadvantages of the prior art. These and other advantages of the present invention will be apparent from the description as set forth below.
In accordance with the instant invention, a filter for analyzing gas (e.g., ambient air or chemically contaminated air) is provided comprising a supported porous polycarbonate membrane, wherein the support is secured, e.g., thermally bound, to the polycarbonate membrane. Preferably, the filter has a pore size of about 1 micrometer (xcexcm) or less, more preferably, about 0.5 micrometers or less.
Filters according to embodiments of the instant invention are suitable for a variety of gas analyzing and/or sampling protocols, including EPA and NIOSH protocols. In preferred embodiments, the filters are suitable for non-gravimetric analysis and sampling protocols, especially for monitoring submicron particulate matter in gas.
In more preferred embodiments, the filter comprises a porous polycarbonate membrane having an annular support secured to the periphery of the membrane, and this configuration allows the filter to be handled easily, while providing a firm gasketing (e.g., sealing) area when the filter is placed in a filter device such as a filter holder, a particle size separator (e.g., an impactor), and/or a filter cassette.